The image formed by a lens would ideally be uniform in brightness over the entire field of view. In practice, most lenses suffer some amount of darkening at the image edges, corresponding to larger input scene field angles θ. This non-uniform field illumination of the image formed depends on the lens structure, but most multi-element or compound lenses can suffer from some degree of such image non-uniformity. An ideal thin lens has an intensity drop-off that depending on field angle as cos4(θ). The source of this drop-off may be a product of extrinsic cos(θ) factors indicating the orientation and distance of the object from the lens, and the orientation of the image sensor surface relative to the lens, and the light energy collected by the lens aperture or entrance pupil. One of the contributions to this reduction in brightness comes from the tilt of the lens entrance aperture relative to the object. For example, a circular thin single lens aperture tilted at 60° collects half as much of the total light energy it collects at 0°. For multiple element lenses, the area of light collection is limited by a physical aperture stop inside the lens. This may typically be caused by a variable diameter iris within the lens, or by the edges of one of the lens elements. The apparent input aperture as seen from the object side of the lens, called the entrance pupil, is the projection of the physical aperture stop within the lens.